The present invention relates to the deposition of thin films on a substrate, particularly to a method for producing thin or multilayer films with uniform or graded thickness, and more particularly to the use of dynamic masking for producing ion beam sputtered or evaporated thin or multilayer films with high thickness uniformity or thickness gradients.
In recent years substantial effort has been directed to the development of thin films, particularly multilayer films or coatings. Thin films or coatings are produced by various vapor-deposition techniques in which a substrate to be coated is exposed to a vapor of the coating material and accumulates a thin film thereon through condensation of the vapor. In the case of optical films for extreme ultraviolet (EUV) lithography, it is desirable that the coating be very uniform (˜0.1%) in thickness across the substrate. Also, there are optical applications which require film or coating thickness gradients.
Multilayer coatings for EUV lithography masks and other optical coatings are commonly deposited with ion beam sputtering, as shown in FIG. 1. The typical system contains a focused ion source directed at a sputter target. The ions sputtered from the material of the target are directed onto a substrate. For deposition of multilayers, the ion beam is interrupted by either turning off the ion flux or shuttering the ion beam, during which time a sputter target with a different composition is interchanged with the original target.
Another common vapor deposition technique is evaporation, in which the source material is heated so that vapor is emitted which condenses on the substrate. The source can be heated with an electron beam, resistively, or by other means. The flux from the evaporation source is similar to that from the ion beam sputter target.
Modification of film properties deposited with ion beam sputtering and evaporation can be achieved by directing a separate ion source at the substrate, as shown in FIG. 2A for ion beam sputtering and in FIG. 2B for e-beam evaporation. The ion assist may remove some of the deposited species, so it must also be controlled to achieve precise thickness control.
In these prior techniques, the locations, separation distances, and tilt angles of the components are adjusted to trade off performance variables such as deposition rate and thickness distribution (uniformity or gradient) and defect levels. However, the effects of the system configuration on the performance variables and their trade-offs are complex and not well understood. The situation is even more complex for curved optical substrates.
The film thickness distribution is most strongly affected by the source-to-substrate distance and tilt angle of the substrate. The substrate(s) are commonly spun either about their own axis of symmetry or another axis to improve azimuthal uniformity, but radial non-uniformities remain. Carefully shaped deposition masks, or shapers, can be inserted between the source and the substrate to compensate for non-uniformity. The masking operation requires tedious iteration of the shape of the mask, and can be impractical for cases where very high uniformity is required. Since the source flux distribution can change over time as the source or target is consumed, the mask shape may require modification. This would require another mask fabrication and perhaps venting the chamber to atmosphere (undesirable) to install the new mask.
In addition to the need for uniform thickness of thin single layer or multilayer films, there is also a need for thin single layer and multilayer films of graded thickness. One prior approach to obtaining graded thickness involves computer-controlled movement of shutters. See M. P. Bruijn, et al., Automatic electron-beam deposition of multilayer soft x-ray coatings with laterally graded d-spacing, Optical Engineering, August 1986, Vol. 25, No. 8. In this approach, two shutters are moved in mutually perpendicular directions in front of a substrate over an area 10 cm×10 cm, with each shutter having a linear motion, such that movement of the two perpendicular slits result in a square aperture through which flux from a source is directed onto the substrate. A recently successfully demonstrated technique which greatly improves uniformity for magnetron sputter deposition systems is described and claimed in copending U.S. Pat. No. 6,524,449, filed Dec. 3, 1999, issued Feb. 25, 2003, entitled “Method and System For Producing Sputtered Thin Films With Sub-Angstrom Thickness Uniformity or Custom Thickness Gradients,” assigned to the same assignee, and is directed to systems in which the substrate is translated across stationary sources. That technique involves measuring the non-uniform flux distribution from the sputter sources—the flux distribution is then used as input data to a computer model that relates a given velocity profile of the substrate platter to the resulting thickness distribution of the deposited films. With a set of these relationships calculated in advance, the user can select a substrate platter velocity profile recipe to obtain the desired film thickness distribution. The method of the above-referenced application has been successfully used to improve thin film uniformity to 0.1% over 6-inch substrates and on curved optics (a 5× factor of improvement). Furthermore, that method allows one to develop the process more rapidly than the purely empirical approach, with about half the number of process development runs (iterations). This is especially valuable when coating sets of optics—the many different sizes, shapes, and prescriptions require development of a customized process for every different optic.
The present invention provides a modification to vapor deposition systems, which enables deposition of highly uniform or graded-thickness thin or multilayer films. The invention involves a method which utilizes a moving shaper, or dynamic mask, which blocks some of the flux from the sputter target or evaporation source before it deposits on the substrate. The acceleration, velocity and position of the mask are computer controlled to precisely tailor the film thickness distribution.